Tires and other rubber goods are employed in many applications that require dynamic deformations. Thus, a relative amount of energy that is stored or lost as heat during the deformations, known as “hysteresis”, is important in assessing the performance of the rubber product. While the polymer and various ingredients play a significant role in hysteresis, the carbon black is also a very important contributor. Carbon blacks that can impart less energy loss in the form of heat at the same level of reinforcement as similar grade carbon blacks are known as “low hysteresis” carbon blacks.
Increasing energy prices are forcing both consumers and manufacturers to look for better technologies in order to conserve energy, keep costs under control, and preserve as much as possible of non-renewable natural resources. The above statement also applies to tire manufacturers. Development of new, less hysteretic materials for tire applications is one of the important focus areas for each tire manufacturer.
Tires are an integral part of the energy conservation process. Experiments and theoretical calculations have shown that as much as seventy percent of the rolling resistance of a tire originates from the tread region, and the remaining thirty percent from other tire components. Since filled rubber compounds typically contain a significant amount of filler, the filler influence on the compound's hysteresis is considerable.
When developing new fillers such as carbon black for tire applications, one has to be mindful of the many performance parameters affected by the filler as well as many basic functions of the pneumatic tire. The tire must provide load carrying capacity, transmit driving and braking torque, provide steering response, produce cornering forces and minimize road noise and vibrations. In addition, it must resist abrasion, have low rolling resistance and, of course, be durable and safe. Furthermore, the tire has many components which contain different types of rubber compounds to optimize the properties for that specific part of the tire. Typical components of a tire include the tread, sidewall, shoulder, bead, plies, belts, liner and chafer. For the new carbon blacks described herein, we have chosen to simply classify them as tread and non-tread. In designing a new carbon black for any of these components, one has to understand not only its function but also where improvements can be made.
The tread section of a tire is where contact between the road and the vehicle is made. The rubber in this area must be compounded to provide a right balance of wear resistance, heat buildup or hysteresis, and traction. The tread is normally a blend of different synthetic rubbers and/or natural rubber. It also contains carbon black, oil, curatives, antioxidants and additional chemicals for processing and performance optimization. Depending on the application, the balance of wear resistance, hysteresis and traction is modified through the different raw materials with carbon black being one of the most critical components. The new chemically treated tread grade of carbon black presented herein is for applications in which hysteresis and abrasion resistance are given more priority in the balance.
In the non-tread sections of the tire, there is concern about many other performance properties of the rubber compound that affect the tire's durability, ride comfort, handling, etc. The sidewall is the portion of the tire between the beads and the tread that control the ride and offer support. The sidewall is usually compounded to give high flexibility and weather resistance. The shoulder of the tire is the upper portion of the sidewall just below the edge of the tread. The shoulder is critical for tire properties such as cornering and heat development thus the rubber in this section is compounded to optimize these characteristics. The bead is a structure composed of high tensile strength; bronze plated steel wire that is wound to form a continuous strand that is coated with rubber. The bead functions as an anchor for the plies and holds the tire on the rim of the wheel. Each steel wire is encapsulated by a special rubber compound that helps in distributing stresses uniformly among the wires and combines them into a strong flexible component. The carcass plies are rubber coated layers of fabric cord that extend from bead to bead reinforcing the tire. The rubber compound for the plies is spread onto the fabric in an operation known as calendering. The rubber compound for this section is designed to provide little or no shrinkage, tackiness appropriate for the adhesion to the fabric, and proper viscosity so that it can be applied uniformly. The belts are directly under the tread and consist of tire cord (fabric or steel) imbedded into a rubber compound whose function is to restrict or hold the carcass plies and to help resist deformation in the tire footprint. The liner is the innermost layer of rubber. It is a thin layer that prevents the compressed gas inside a tire from diffusing through the various rubber components. This is obviously a special compound that must have low permeability to gases used in various tires but also other properties such as good thermal/oxidative stability and good adhesion to the body stock. The chafer is a narrow strip of yet another rubber compound that protects the cord from the rim and helps distribute flex above the rim.
The point of reiterating all these functional components of a tire is to illustrate that they require many special compounds and that each plays a role in the performance of the tire as a whole. Developing a combination of properties in any part of the tire that leads to an overall improvement in the tire's hysteresis performance is very advantageous and beneficial.
Most of the carbon black used in the tire industry is produced using the furnace process. In the furnace process, the feedstock is incompletely combusted in a reaction to form a very finely divided material composed of aggregates that are the carbon black monounits. These aggregates are typically submicron in size and of very complex structure. The surface of the aggregates may be covered with turbostratic graphitic crystallites and areas of disorganized amorphous carbon.
In order to produce different grades of carbon black, different reactor technology is sometimes employed. As a general rule, tread grade blacks are produced using different reactor design as compared to carcass grades. In addition to specific reactor design, the number and position of oil spraying nozzles, the ratio of air to feedstock and natural gas, as well as the position of the quench water sprays also plays a critical role in setting the carbon black properties (size, surface area, and structure). When carbon black is formed in the reactor, which takes only a few milliseconds, it is in the form of a thick smoke with apparent density of about 10−2 g/cm3. The remaining production time, which could be up to two hours, is conveying and handling the product in order to prepare the carbon black to be shipped and further processed.
Due to the low density of the products, it is necessary to densify the material. This process is usually achieved through a pelletization. The finished pellets are quasispherical with a diameter in the millimeter range and an apparent density of about 0.35 g/cm3.
Tread grade carbon blacks such as 100- and 200-series, e.g., N134 and N234 tread grade carbon blacks, respectively, are common commercial grades used in the production of tires. While delivering satisfactory performance in many applications, they are not capable of meeting many of the new demands for lower hysteresis blacks. Many grades have been developed to improve hysteresis over these types of grades, but typically there is a tradeoff of properties. For example, wide aggregate size distribution carbon blacks have had some success in achieving lower hysteresis but do so with some loss in treadwear.
There is a continuing need for a carbon black product that provides an improved hysteresis over conventional carbon black products when incorporated into rubber compounds. Desirably, the treated carbon black product provides the improved hysteresis when used in tire tread rubber compounds, without undesirably affecting treadwear of the tire tread rubber compounds.